This setting is in the ad manager. If you're looking for a certain nationalistic feel you should also target people that are politically active. Facebook offers this option as well for certain countries.

Technically yes, you can. But would always advise a more targeted approach to advertising on Facebook, or any marketing channel. Although you product may be nationalistic, just because someone is Irish doesn't mean they would have an interest in buying it. You'd have to be a certain type of person. Would strongly advise more thought around the audience you are trying to reach and what motivates them along with thinking about your current customers to work out a customer profile. Then developing a highly targeted ad campaign around that with creative and copy that will resonate with your potential customer. Hope this helps, let me know if you'd like to talk more.